Proposals have already been made to perform such color determination by illuminating the surface of the object or of the tooth with polychromatic light flux, by picking up the light backscattered by said object or said tooth, and by analyzing the spectrum of the light picked up, in particular to determine the spectral reflectance of the object or of the tooth.
In theory, knowledge of such spectral reflectance makes it possible without risk of error and in particular without risk of metamerism to determine the color that should be given to a dental prosthesis that is implanted next to the tooth on which the measurement has been performed, or the color to be given to a series of objects reproducing the object on which the measurements have been performed.
Nevertheless, in practice, it turns out that there are several difficulties in implementing that known technique which lead to relatively large color differences between a tooth in the mouth and a dental prosthesis between an object and the reproductions of said object.
These differences are due, firstly, to the sensitivity characteristics of the human eye, and secondly to the way in which teeth or objects that are to be reproduced are transparent, diffusing, and absorbent to greater or lesser extents.
These difficulties are further aggravated when manufacturing dental prosthesis since natural teeth are generally more transparent than artificial teeth.
To solve these problems, the Applicant has already developed a method of determining the color of a transparent or translucent object, e.g. a tooth in the mouth, as disclosed and protected by U.S. patent application Ser. No. 07/991201 filed on Dec. 15, 1992 the contents of which is incorporated herein by reference.
In summary, the method described in that patent application consists essentially in measuring the reflectance of the object or of the tooth at wavelengths that correspond to sensitivity maxima in the human eye with respect to hue and to saturation, and to deduce from such measurements an indicator for reproducing the transparency of the object, together with a saturation coefficient for the color of the object as seen by the human eye, and the pigmentations of said color relative to white in predetermined wavelength intervals.
It is preferable, particularly when the object is a tooth in the mouth, to use an illuminant which is the standardized diffuse illumination of sunlight, the reflectance of the tooth is measured at wavelengths substantially equal to 440 nm, 495 nm, 520 nm, 577 nm, and 600 nm, and the pigmentations of the color of the object relative to white are determined over wavelength intervals lying respectively between about 400 nm to 550 nm and between about 550 nm and 650 nm.
That method makes it possible to determine the color of a tooth exactly, and then to compare said determined color with known colors of a color chart or palette by comparing the saturation percentage of the tooth color with the corresponding saturation percentages of the colors in the chart or palette, and by comparing the pigmentations of the tooth color with the corresponding pigmentations of the colors of the chart or palette, and finally by comparing the luminance of a white object included in the luminance of the tooth with the luminances of a white object included in the luminances of the colors in the chart or palette.